Adjustable impedance for use in waveguides



y 1950 F. c. DE RONDE 2,944,234

ADJUSTABLE IMPEDANCE FOR USE IN WAVEGUIDES Filed June 17, 1957 INVENTQRFRANS CHRISTIAAN DE RONDE AGEN United States Patent 2,944,234 ADJUSTABLEIMPEDANCE FOR USE IN WAVEGUIDES Frans Christiaan de Ronde, Eindhoven,Netherlands, assignor to North American Philips Company, Inc., New York,N.Y., a corporation of Delaware Filed June '17, 1957, Ser. No. 666,058Claims priority, application Netherlands July 11, 1956 1 Claim. (Cl.33398) This invention relates to adjustable impedances for use inwaveguides. Such impedances may, for example, be employed forterminating a waveguide in a reflectionfree manner or as a comparisonimpedance in a measuring arrangement for measuring unknown impedances.With known adjustable impedances, the modulus and the argument of thereflection factor are, in general, not adjustable independently of eachother and these impedances are not so well suited for use in wavesystems for millimetre waves.

The present invention provides a simple adjust-able impedance forwaveguides, in which the modulus and the argument of the reflectionfactor are adjustable independently of each other and which isparticularly suitable for use in measuring arrangements, since themodulus and the argument are simple functions of two variable factors,so that the impedance can be calibrated in an absolute manner. With theadjustable impedance according to the invention, the waveguide comprisesa first flat strip, which consists at least at its surface of resistivematerial and is fixedly positioned in a plane extending through the axisof the waveguide. Furthermore, a second fla-t strip, which also consistsof resistive material at least at its surface, is secured at rightangles to the surface of a short-circuiting piston which, together withthe second strip, is rotatable and also is movable in an axial directionof the waveguide.

A variable attenuator is known per se, in which two flat strips ofresistive material are fixedly arranged in a waveguide and, betweenthese strips, a third strip of resistive material is rotatable about theaxis of the waveguide.

This device permits only the value of the attenuation tobe controlled,but the phase of the waves is not variable.

In order that the invention may be readily carried into effect, anexample will now be described in detail with reference to theaccompanying drawing, in which Fig. 1A shows a circular waveguide G inwhich a strip S1 is arranged in a plane through the axis of the waveguide. The strip may, for example, consist of a mica plate which isprovided with a layer of resistive material, for example finely dividedcarbon. The waveguide G contains a short-circuiting piston Z which, inorder to prevent the emission of energy, is in known manner composed ofthree parts Z1, Z2 and Z3, the length of which corresponds to onequarter wavelength. Onthe left-hand surface C of the piston Z restsanother strip S2 which may also consist of a mica plate provided with alayer of resistive material. The strip S2 extends at right angles to thesurface of the piston in a plane through the axis of the piston. Thepiston Z together with'the strip S2 is rotatable about the axis of thewaveguide and also is movable in a longitudinal direction of thewaveguide. The position of the piston is read off scale divisions (notshown). In order to reduce reflection of the waves against the strips S1and S2, the ends of the strips are bevelled (see strip S2).

Fig. 1B is a cross-sectional view of the waveguide on the line 1B-1B.The left-hand end of the waveguide G may, for example, through a gradualtransition, be connccted to a wave-guide of rectangular cross-section.

This device operates as follows:

The incident waves at the input B are linearly polarized at right anglesto the strip S1, as indicated by a vector E in Fig. 2. Hence, theycanpass unhindered through the strip Si. The strip S2 subtends, forexample, an angle a with the direction of polarization of the incidentwaves E. The incident waves E may be thought of as being decomposed intotwo components, a first component Ea, the sense of polarization of whichis parallel to the plane of the strip S2, and a component Eb, the senseof polarization of which is at right angles to the plane of the stripand the strength of which is equal to E sin a. The first component Ea isentirely absorbed by the strip S2. The component Eb, however, passesunhindered through the strip S2 and is completely reflected by thepiston Z. The reflected component may in turn be decomposed into twocomponents, that is to say a component Be, the sense of polarization ofwhich is parallel to that of the strip S1, and a component Ed, the senseof polarization of which is at right angles to that of the strip S1 andconsequently corresponds to the sense of polarization of the incidentwaves E. The component E0 is fully absorbed by the strip S1. However,the component Ed will pass unhindered through the strip S1 and emergesto the left. The strength of this component corresponds to Eb sin a=Esin a. By varying the angle a, the strength of the reflected wave, thatis to say the modulus sin a of the reflection coefiicient isconsequently adjustable to any desired value in a simple manner.

The phase difference between the incident waves E and the reflectedwaves Ed corresponds to twice the spacing between the inlet B and thesurface C of the piston Z expressed in electric degrees plus since aphase change of 180 occurs upon reflection against the piston surface.The phase difference between the incident waves E and the reflectedwaves Ed, that is to say the argument of the reflection factor, isconsequently adjustable to any desired value by moving the piston Zeither to the left or to the right.

The device may, for example, *be employed for calibrating unknownimpedances by means of a so-called magic- T, the unknown impedance andthe variable impedance being, in accordance with the invention,connected to arms opposite each other of the magic-T, a third am beingconnected to a generator, and a fourth am being connected to a measuringdevice. The variable impedance is varied in such manner that no waveenergy appears in the fourth arm. In this case, the unknown impedance isequal to the variable impedance, and the modulus and the argument can beread oil the scale-division of the variable impedance.

What is claimed is:

A variable impedance arrangement comprising a hollow waveguide, a firstflat strip composed at least in part of resistive material positioned insaid waveguide in a plane passing through the axis of said Waveguide, ashortcircuiting piston positioned in said waveguide and adapted to beindependently rotated and moved in the axial direction of the waveguide,and a second flat strip composed at least in part of resistive materialattached to the end of said piston which faces said first flat strip andbeing rotatable with said piston, said second flat strip being alignedalong the axis of said waveguide.

References Cited in the file of this patent UNITED STATES PATENTS2,088,749 King Aug. 3, 1937 2,603,709 Bowen July 15, 1952 2,692,977Koppel Oct. 26, 1954 2,808,571 Cohn Oct. 1, 1957 FOREIGN PATENTS.

691,939 Great Britain May 27, 1953

